Cache ClassNotFounds in CompositeClassLoader to speed up compilation

Compilation of our rules are taking a long time.
During profiling, one bottleneck that appears is excessive Class.forName lookups.
CompositeClassLoader caches class that are found successfully, but does not cache those that were not found.
Mvel tries to lookup "non-existent" classes a lot during compilation.
Caching these ClassNotFounds should speed up the compilation.

This change alone brought down our compilation time from 163854ms to 47312ms
on my dev box.